About an hour before Donald J. Trump was set to address a large crowd here on Thursday, Brandon Partin, a Trump supporter, draped a Confederate flag over the front rail just to the right of the stage.

But the flag didn’t remain up for long. As Sharon Day, a co-chairman of the Republican National Committee, delivered her introductory remarks, a campaign staff member and the local police moved to remove the flag.

Three police officers stood on the floor tugging gently at the flag, indicating that it had to come down. Mr. Partin, 27, grew irritated that he had to remove it. He had just purchased the flag outside the event, and “Trump 2016” was printed on it.